1. Field of the Invention
The present invention relates to an engine control system for a construction machine, and more particularly to an engine control system for a construction machine such as a hydraulic excavator wherein a diesel engine having an electronic fuel injection device (electronic control governor) is used as a prime mover.
2. Description of the Related Art
A construction machine such as a hydraulic excavator generally includes at least one hydraulic pump for driving a plurality of actuators, and a diesel engine is used as a prime mover for rotatively driving the hydraulic pump. The diesel engine is controlled in injected fuel amount and fuel injection timing by a fuel injection device. Of them, the fuel injection timing has been conventionally determined by a mechanical timer mechanism depending on a revolution speed in most cases. With recent development of electronic control in the fuel injection device, however, the fuel injection timing has become freely controllable by an injection timing control actuator in addition to the injected fuel amount. As a result, good combustion is realized and engine performance is improved in a wide range by determining the optimum injection timing depending on a status variable such as the engine revolution.
For example, JP, A, 1-110839 discloses an internal combustion engine with a turbocharger wherein an intake pressure is detected by a pressure sensor at the time of quick acceleration to control the fuel injection timing such that the timing is advanced a predetermined angle to reduce the generation of black smoke when the detected intake pressure is not higher than a setting reference value, and is not advanced to prevent an abnormal rise of pressure in a cylinder when the detected intake pressure is not lower than the setting reference value. Also, FIGS. 1 and 2 of the Publication show that an engine load is input as one item of information to be reflected in control of the injection timing.
On the other hand, earlier timing of fuel injection provides a higher combustion temperature of fuel injected into a cylinder and hence better fuel efficiency (fuel consumption). As stated in, e.g., "Mechanization of Construction" (December 1996 No. 562), an article titled "Overview and Inspection/Servicing of Diesel Engine Adapted for Exhaust Gas Regulation (No. 2)", page 63, however, NO.sub.x meaning NO and NO.sub.2 together, which are said to be responsible for photochemical smog, generally tends to be produced during operation at a high speed and under a high load. To make exhaust gas clean, therefore, a method of delaying the fuel injection timing in a high-speed and high-load condition, where NO.sub.x tends to be produced, is employed.